The article proposes a refined Euler-Lagrange mathematical model of the separation process of emulsified petroleum products and solid suspensions from atmospheric surface effluents in coalescent separators with oleophilic polymer corrugated plates. There has been carried out a numerical experiment using a model of the hydrodynamics of flow in the gaps between the plates. The adequacy of the mathematical model is confirmed by experiments on real environment. The calculation results obtained according to the developed mathematical model are the same as those obtained in the course of experiments with an accuracy of ± 9%. The model can be used to calculate coalescent separators designed to purify storm water from dispersed particles and oil droplets.